The present invention relates to a method and apparatus for conducting offshore well operations which include exploratory drilling and which also include production operations from the same platform.
Generally speaking, offshore well operations have employed a semisubmersible vessel or platform which provides relatively great mobility and which can be located over a proposed well site, anchored by catenary temporary moorings, and moved into position for drilling exploratory well holes to determine the presence and character of a field of oil or hydrocarbons. Such semisubmersible platforms are generally designed to operate with horizontal buoyant members located beneath the influence of wave action so that the semisubmersible vessel does not substnatially react to wave influence. Exploratory drilling is conducted from such a semisubmersible vessel and is feasible by the use of various types of heave compensating means. Heretofore, after exploratory drilling was completed, if the exploratory well was not suitable for production, the semisubmersible vessel was readily moved to another well site. If the exploratory well could be produced, the well site was marked, recorded and preparations commenced to bring a permanent production platform to the site. Construction and outfitting of such a production platform was extremely costly and required a long period of time, in some instances as much as six or seven years elapsed before a production platform could be brought to the well site, installed, and production operations begun.
Prior semisubmersible platforms embodied a design of vertical and horizontal buoyant members which enhanced the mobility of the semisubmersible platform, which were operable with relatively low profile at the water surface, and which utilized catenary mooring lines held by temporary anchors at the sea floor and located a distance from the well site. A tension leg platform embodies a different hydrodynamic design and is superior for field development and production because the design characteristics eliminate heave, pitch and roll of the platform. Thus, a tension leg platform is quite stable and can be a relatively permanent installation. Since the hydrodynamic design of a tension leg platform is different than the design of a semisubmersible platform, the conduct of offshore well operations has heretofore generally required the use of two different vessels for exploration and for production. Tension leg platform design has been optimized for minimum variation of mooring tension, whereas semisubmersible vessel design has been optimized for minimum heave response with catenary mooring. Prior semisubmersible constructions and prior tension leg platform constructions have been generally equipped for one or the other type of operation; namely, either exploratory operations or production operations.